Recent years have seen research for using an optical integrated circuit including an optical waveguide element in various devices for optical fiber communication. Examples of devices for optical fiber communication include (i) an optical-transport device for use in long-distance, metro-network wavelength-division multiplexing optical fiber communication and (ii) an optical interconnect device for use at, for example, a data center.
Patent Literature 1 discloses, as an optical waveguide element for use in an optical integrated circuit, an optical waveguide element including a core having (i) two silicon layers having respective electrical conductivities different from each other and (ii) a dielectric layer sandwiched between the two silicon layers. The optical waveguide element disclosed in Patent Literature 1 has a so-called silicon capacitor (SISCAP) structure: It includes a p-type silicon layer and an N-type silicon layer as the two silicon layers and silicon dioxide for the dielectric layer.
The optical waveguide element disclosed in Patent Literature 1 is configured such that (i) light is confined with use of the difference in refractive index between a sidewall of each silicon layer (that is, an end of each silicon layer in the direction of a substrate surface) and the cladding and that (ii) light guided through the core has an electric field that is so distributed as to spread across the two silicon layers and the dielectric layer. Changing the respective carrier densities of the two silicon layers can thus change the refractive index for light guided through the core at a central portion of the core at which central portion the two silicon layers overlap each other with the dielectric layer therebetween.
Patent Literature 1 discloses a SISCAP structure in which either the p-type silicon layer or the N-type silicon layer is bent into an L shape. Such a structure allows a region at which the carrier density is changed to extend not only in a direction parallel to the substrate surface but also in a direction perpendicular to the substrate surface.